Modern photoelastic modulators (PEM) have been used extensively for scientific and technical measurements of optical quantities related to polarization states and optical birefringence. Examples such as strain inside optically transparent materials, dichroism measurements for the understanding of physical-chemical structures of a wide range of bio-medical and chemicals with out-of-symmetry molecular structures while the thickness of thin deposition films is especially important in the semiconductors industry. It is essential to investigate the principles and physical constructions of both designs to reveal the sources of the problems and provides a ground to understand the present invention.
The working principle of the modern photoelastic modulator (PEM) is based on the optical birefringence induced by periodical stress which is the result of acoustic standing waves built-up inside an optically transparent rectangular block, formed from an optically transparent material with a high elasto-optic efficiency and low mechanical dissipation (e.g. fused quartz). A longitudinal acoustic wave can be excited by an external driving source, when the frequency of the source satisfies certain conditions, the wave reflected back from the ends of the block will be constructive and built up a longitudinal standing wave. This standing wave will die down after the excitation source is removed. If the excitation continues (usually, physical contact with the exciting source is made), then a stable standing wave will be established. The periodic strain variation inside the optical block that builds up changes the refractive index of the region where the strain is at a maximum in the optical block with the same periodicity as the excitation. This optical block is then acting as a dynamic wave plate so, if a plane polarized light beam is passed through it at an appropriate angle, it's the beam's state of polarization is changed (or phase modulation) according to the degree of strain that is built up by the excitation of the external source.
The modern PEM was invented in 1969 by Dr. James Kemp (J. C. Kemp, Journal of the Optical Society of America, 1969, vol. 59, pp 950-954), then manufactured by a private company in the USA (Hinds Instruments, Inc., Hillsboro, Oreg., USA). The design has since been dominant in almost all PEM applications, yet the construction and performance have changed little. Its main disadvantage (of being sensitive to temperature and fluctuations in the working environment) has been the major hurdle that limits its applications.
A second type PEM was invented by Canit and Badoz of France in 1983 (Applied Optics, vol. 22, pp. 592, 1983), which utilized a thin slab of piezoelectric ceramic transducer adhered to the narrow side of a rectangular block of optical material (e.g. fused silica). The longitudinal standing (acoustic) wave is excited by the shear coupling of the ceramic transducer, the single block structure almost eliminates the temperature problem in the Kemp design, and is much more efficient in terms of transfer the electrical control signal to the resultant optical modulation.
There are however disadvantages which prevent the design from being widely used. Firstly, “shear coupling” introduces a series of complicated vibrations besides the longitudinal wave and, secondly, to make the shear coupling efficient, the thickness of the PEM body tends to be too thick, and makes the residual strain inside the optical block two to four times as much as the Kemp design.
In the PEMs described in the previous paragraphs, the fundamental acoustical oscillation frequency is the full wavelength mode, and the optical modulation is at the corresponding frequency. It is important to excite the fundamental mode with minimum unwanted perturbations. The Kemp design, utilizes a −18.5 degree X-cut quartz crystal as the excitation source, which delivers a nearly pure longitudinal wave that satisfies such a requirement. In the Canit-Badoz design, the shear coupling of the thin ceramic transducer induces a series of unwanted waves. A whole spectrum of acoustic vibrations was detected in the frequency domain, with resonance peaks spread over a wide spectral range. Although, the resonance peaks are positioned in different frequencies, and are transient, the waves and reflections will upset the purity of optical modulation.